Remove `akka-stream-testkit-experimental` file ghosting redundencies

[carlpulley]

Currently, the package akka-stream-testkit-experimental is an empty package. Hence, in order to write Akka reactive streaming tests, we need to explicitly include the relevant (ghost) source files in our project. This will obviously change in due course!

This issue exists to:

• track the package akka-stream-testkit-experimental
• ensure code that is currently ghosted in server/exercise/src/test/scala/akka/stream/testkit/ is deleted once this package is formally "released".